Organic acids are key metabolites for diverse fruit flavoring and vital for fruit quality measures. Their contents are significantly varied regarding the canopy position. However, the underlying mechanism is still unclear. In this study by using Citrus raticulata cv. Bendizao as research material, a significant 2-fold increase in citrate content and a 2-fold decrease in malate content were noticed in bottom-canopy fruits subjected to less light and low temperature compared to top-canopy fruits. Moreover, transcript levels of genes like citrate synthase (CS1), aconitate hydratase genes (ACO1 and ACO2), NAD-dependent malic enzyme gene (NAD-ME) and malate dehydrogenase genes (MDH1 and MDH2) were significantly higher, while transcript levels of other genes such as citrate utilization genes (ACL2, ACL3 and ACO3), malate synthase gene (MS) and cytosolic NAD-dependent malate dehydrogenase gene (cytNAD-MDH) were significantly lower in bottom-canopy fruits compared to top-canopy fruits. Additionally, transcript levels of proton pump genes (CsPHs and VHA), two transporter genes (MATE and ALMT9), and four transcription factor genes (MYB73, Noemi, MYC-N, and bHLH) were significantly higher, while mRNA levels of two transcription factor genes (MYB10 and WRKY14) and transporter genes (CsCit1 and ALMT9-like) were significantly lower in bottom-canopy fruits than those in top-canopy fruits. Taken together, the variance of organic acid profiles in the canopy could be attributed to the microclimate change, which comprehensively influences the organic acid accumulation-related genes’ transcript levels; reduced malate accumulation in bottom-canopy fruits is mainly due to the minimal expression of MS and cytNAD-MDH, and high expression of malate utilization genes (MDH and NAD-ME), while the higher citrate content in the bottom-canopy fruits is probably due to the enhanced vacuolar storage through increasing vacuolar proton pump genes and MATE transcript levels, and the minimal citrate degradation and efflux through decreasing ACL2, ACL3, ACO3 and CsCit1 transcript levels.

有机酸是多种水果调味的关键代谢物，对水果质量测量至关重要。关于冠层位置，它们的内容有很大差异。然而，潜在的机制仍不清楚。在这项研究中，使用Citrus raticulata cv。作为研究材料的本底早，与顶冠果实相比，在光照和低温条件下，底冠果实的柠檬酸盐含量显着增加了2倍，苹果酸含量显着降低了2倍。此外，柠檬酸合酶 ( CS1 )、乌头酸水合酶基因 ( ACO1和ACO2 )、NAD 依赖性苹果酸酶基因 ( NAD-ME ) 和苹果酸脱氢酶基因 ( MDH1和MDH2）显着升高，而其他基因如柠檬酸利用基因（ACL2、 ACL3和ACO3）、苹果酸合酶基因（MS）和细胞溶质 NAD 依赖性苹果酸脱氢酶基因（cytNAD-MDH）的转录水平显着降低。冠层果实与顶冠果实相比。此外，质子泵基因（CsPHs和VHA）、两个转运蛋白基因（MATE和ALMT9）和四个转录因子基因（MYB73、Noemi、MYC-N和bHLH）的转录水平显着升高，而两个转录基因的 mRNA 水平显着升高。因子基因（MYB10和WRKY14）和转运蛋白基因（CsCit1和ALMT9-like）在底部冠层果实中显着低于顶部冠层果实。综合来看，冠层有机酸谱的变化可归因于小气候变化，综合影响有机酸积累相关基因的转录水平；底部冠层果实中苹果酸积累减少的主要原因是MS和cytNAD-MDH的最低表达，以及苹果酸利用基因（MDH和NAD - ME ）的高表达。)，而底部冠层果实中较高的柠檬酸盐含量可能是由于通过增加液泡质子泵基因和MATE转录水平来增强液泡储存，以及通过降低ACL2、ACL3、ACO3和CsCit1转录水平来最小化柠檬酸盐降解和流出.